<s>
A	O
bolometer	B-Application
is	O
a	O
device	O
for	O
measuring	O
radiant	O
heat	O
by	O
means	O
of	O
a	O
material	O
having	O
a	O
temperature-dependent	O
electrical	O
resistance	O
.	O
</s>
<s>
A	O
bolometer	B-Application
consists	O
of	O
an	O
absorptive	O
element	O
,	O
such	O
as	O
a	O
thin	O
layer	O
of	O
metal	O
,	O
connected	O
to	O
a	O
thermal	O
reservoir	O
(	O
a	O
body	O
of	O
constant	O
temperature	O
)	O
through	O
a	O
thermal	O
link	O
.	O
</s>
<s>
Metal	O
bolometers	B-Application
usually	O
work	O
without	O
cooling	O
.	O
</s>
<s>
Today	O
,	O
most	O
bolometers	B-Application
use	O
semiconductor	O
or	O
superconductor	O
absorptive	O
elements	O
rather	O
than	O
metals	O
.	O
</s>
<s>
These	O
devices	O
can	O
be	O
operated	O
at	O
cryogenic	B-Algorithm
temperatures	O
,	O
enabling	O
significantly	O
greater	O
sensitivity	O
.	O
</s>
<s>
Bolometers	B-Application
are	O
directly	O
sensitive	O
to	O
the	O
energy	O
left	O
inside	O
the	O
absorber	O
.	O
</s>
<s>
For	O
this	O
reason	O
they	O
can	O
be	O
used	O
not	O
only	O
for	O
ionizing	O
particles	O
and	O
photons	B-Application
,	O
but	O
also	O
for	O
non-ionizing	O
particles	O
,	O
any	O
sort	O
of	O
radiation	O
,	O
and	O
even	O
to	O
search	O
for	O
unknown	O
forms	O
of	O
mass	O
or	O
energy	O
(	O
like	O
dark	B-Application
matter	I-Application
)	O
;	O
this	O
lack	O
of	O
discrimination	O
can	O
also	O
be	O
a	O
shortcoming	O
.	O
</s>
<s>
The	O
most	O
sensitive	O
bolometers	B-Application
are	O
very	O
slow	O
to	O
reset	O
(	O
i.e.	O
,	O
return	O
to	O
thermal	O
equilibrium	O
with	O
the	O
environment	O
)	O
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
compared	O
to	O
more	O
conventional	O
particle	B-Algorithm
detectors	I-Algorithm
,	O
they	O
are	O
extremely	O
efficient	O
in	O
energy	O
resolution	O
and	O
in	O
sensitivity	O
.	O
</s>
<s>
They	O
are	O
also	O
known	O
as	O
thermal	B-Application
detectors	I-Application
.	O
</s>
<s>
The	O
first	O
bolometers	B-Application
made	O
by	O
Langley	O
consisted	O
of	O
two	O
steel	O
,	O
platinum	B-Operating_System
,	O
or	O
palladium	O
foil	O
strips	O
covered	O
with	O
lampblack	O
.	O
</s>
<s>
By	O
1880	O
,	O
Langley	O
's	O
bolometer	B-Application
was	O
refined	O
enough	O
to	O
detect	O
thermal	O
radiation	O
from	O
a	O
cow	O
a	O
quarter	O
of	O
a	O
mile	O
away	O
.	O
</s>
<s>
Nikola	O
Tesla	O
personally	O
asked	O
Dr.	O
Langley	O
if	O
he	O
could	O
use	O
his	O
bolometer	B-Application
for	O
his	O
power	O
transmission	O
experiments	O
in	O
1892	O
.	O
</s>
<s>
While	O
bolometers	B-Application
can	O
be	O
used	O
to	O
measure	O
radiation	O
of	O
any	O
frequency	O
,	O
for	O
most	O
wavelength	O
ranges	O
there	O
are	O
other	O
methods	O
of	O
detection	O
that	O
are	O
more	O
sensitive	O
.	O
</s>
<s>
For	O
sub-millimeter	O
wavelengths	O
through	O
millimeter	O
wavelengths	O
(	O
from	O
around	O
200µm	O
to	O
a	O
fewmm	O
wavelength	O
,	O
also	O
known	O
as	O
the	O
far-infrared	O
,	O
terahertz	B-Architecture
)	O
bolometers	B-Application
are	O
among	O
the	O
most	O
sensitive	O
available	O
detectors	O
,	O
and	O
are	O
therefore	O
used	O
for	O
astronomy	O
at	O
these	O
wavelengths	O
.	O
</s>
<s>
To	O
achieve	O
the	O
best	O
sensitivity	O
,	O
they	O
must	O
be	O
cooled	O
to	O
a	O
fraction	O
of	O
a	O
degree	O
above	O
absolute	B-Application
zero	I-Application
(	O
typically	O
from	O
50mK	O
to	O
300mK	O
)	O
.	O
</s>
<s>
Notable	O
examples	O
of	O
bolometers	B-Application
employed	O
in	O
submillimeter	O
astronomy	O
include	O
the	O
Herschel	O
Space	O
Observatory	O
,	O
the	O
James	O
Clerk	O
Maxwell	O
Telescope	O
,	O
and	O
the	O
Stratospheric	O
Observatory	O
for	O
Infrared	O
Astronomy	O
(	O
SOFIA	O
)	O
.	O
</s>
<s>
Recent	O
examples	O
of	O
bolometers	B-Application
employed	O
in	O
millimeter-wavelength	O
astronomy	O
are	O
AdvACT	O
,	O
BICEP	O
array	O
,	O
SPT-3G	O
and	O
the	O
HFI	O
camera	O
on	O
the	O
Planck	O
satellite	O
,	O
as	O
well	O
as	O
the	O
planned	O
Simons	O
Observatory	O
,	O
CMB-S4	O
experiment	O
,	O
and	O
LiteBIRD	O
satellite	O
.	O
</s>
<s>
The	O
term	O
bolometer	B-Application
is	O
also	O
used	O
in	O
particle	O
physics	O
to	O
designate	O
an	O
unconventional	O
particle	B-Algorithm
detector	I-Algorithm
.	O
</s>
<s>
The	O
bolometers	B-Application
are	O
sensitive	O
not	O
only	O
to	O
light	O
but	O
to	O
every	O
form	O
of	O
energy	O
.	O
</s>
<s>
The	O
operating	O
principle	O
is	O
similar	O
to	O
that	O
of	O
a	O
calorimeter	B-Algorithm
in	O
thermodynamics	O
.	O
</s>
<s>
In	O
the	O
jargon	O
of	O
high	O
energy	O
physics	O
,	O
these	O
devices	O
are	O
not	O
called	O
"	O
calorimeters	B-Algorithm
"	O
,	O
since	O
this	O
term	O
is	O
already	O
used	O
for	O
a	O
different	O
type	O
of	O
detector	O
(	O
see	O
Calorimeter	B-Algorithm
)	O
.	O
</s>
<s>
Their	O
use	O
as	O
particle	B-Algorithm
detectors	I-Algorithm
was	O
proposed	O
from	O
the	O
beginning	O
of	O
the	O
20th	O
century	O
,	O
but	O
the	O
first	O
regular	O
,	O
though	O
pioneering	O
,	O
use	O
was	O
only	O
in	O
the	O
1980s	O
because	O
of	O
the	O
difficulty	O
associated	O
with	O
cooling	O
and	O
operating	O
a	O
system	O
at	O
cryogenic	B-Algorithm
temperature	O
.	O
</s>
<s>
A	O
microbolometer	B-Algorithm
is	O
a	O
specific	O
type	O
of	O
bolometer	B-Application
used	O
as	O
a	O
detector	O
in	O
a	O
thermal	O
camera	O
.	O
</s>
<s>
The	O
microbolometer	B-Algorithm
grid	O
is	O
commonly	O
found	O
in	O
three	O
sizes	O
,	O
a	O
640×480	O
array	O
,	O
a	O
320×240	O
array	O
(	O
384×288	O
amorphous	O
silicon	O
)	O
or	O
less	O
expensive	O
160×120	O
array	O
.	O
</s>
<s>
The	O
hot	O
electron	O
bolometer	B-Application
(	O
HEB	O
)	O
operates	O
at	O
cryogenic	B-Algorithm
temperatures	O
,	O
typically	O
within	O
a	O
few	O
degrees	O
of	O
absolute	B-Application
zero	I-Application
.	O
</s>
<s>
At	O
these	O
very	O
low	O
temperatures	O
,	O
the	O
electron	O
system	O
in	O
a	O
metal	O
is	O
weakly	O
coupled	O
to	O
the	O
phonon	B-Language
system	O
.	O
</s>
<s>
Power	O
coupled	O
to	O
the	O
electron	O
system	O
drives	O
it	O
out	O
of	O
thermal	O
equilibrium	O
with	O
the	O
phonon	B-Language
system	O
,	O
creating	O
hot	O
electrons	O
.	O
</s>
<s>
Phonons	B-Language
in	O
the	O
metal	O
are	O
typically	O
well-coupled	O
to	O
substrate	O
phonons	B-Language
and	O
act	O
as	O
a	O
thermal	O
reservoir	O
.	O
</s>
<s>
In	O
describing	O
the	O
performance	O
of	O
the	O
HEB	O
,	O
the	O
relevant	O
heat	O
capacity	O
is	O
the	O
electronic	O
heat	O
capacity	O
and	O
the	O
relevant	O
thermal	O
conductance	O
is	O
the	O
electron-phonon	O
thermal	O
conductance	O
.	O
</s>
<s>
A	O
bolometer	B-Application
can	O
be	O
used	O
to	O
measure	O
power	O
at	O
microwave	O
frequencies	O
.	O
</s>
<s>
After	O
applying	O
microwave	O
power	O
,	O
the	O
bias	O
current	O
is	O
reduced	O
to	O
return	O
the	O
bolometer	B-Application
to	O
its	O
resistance	O
in	O
the	O
absence	O
of	O
microwave	O
power	O
.	O
</s>
<s>
The	O
average	O
response	O
time	O
of	O
the	O
bolometer	B-Application
allows	O
convenient	O
measurement	O
of	O
the	O
power	O
of	O
a	O
pulsed	O
source	O
.	O
</s>
<s>
In	O
2020	O
,	O
two	O
groups	O
reported	O
microwave	O
bolometers	B-Application
based	O
on	O
graphene-based	O
materials	O
capable	O
of	O
microwave	O
detection	O
at	O
the	O
single-photon	O
level	O
.	O
</s>
